Design,Preparation And Photoelectrochemical Properties Of α-Fe₂O₃ Films

Converting solar energy in the form of chemical fuels via photoelectrochemical route is a promising technique for clean renewable energy.Photoelectrochemical reactions such as producing hydrogen from water or converting CO2 into CO or CH3OH require a low-cost source of electrons where water could be the most promising candidate.Due to the sluggish kinetics of oxidizing water into oxygen,water oxidation is the bottleneck of photoelectrochemical fuel conversion and low-cost,high conversion efficiency photoanode is the key.Hematite(α-Fe2O3,band gap of 2.1 eV)is considered as the most promising photoanode in PEC water oxidation for it is cheap,environmental friendly,abundant and chemically stable.Specifically,itstheoretical efficiency of solar to hydrogen is as high as 14～16%.However,poor conductivity,short hole diffusion length and sluggish water oxidation kinetics have resulted the low observed STH value than the theoretical one.To improve PEC performance of α-Fe2O3,this dissertation focuses on structural design and chemical modification of α-Fe2O3 films and investigations on the corresponding mechanisms.The main conclusions drawn from the works are as follows.(1)In order to elucidate the effect of doping on the PEC performance of hematite films,transition metal elements doped hematite films were obtained from thermal oxidation of magnetron sputtered alloy films.The PEC performance of hematite and the effect of doping on the film conductivity,oxygen evolution kinetics,lifetime of photocarriers,hole diffusion length were achieved by modulating the doping element,doping level and film thickness.Eventually,the J and Von were optimized.The results indicated that the α-Fe2O3 films were consisted of 150-200 nm nanoparticles regardless of the doping element.The band gap almost kept constant or shifted slightly.Because of the difference ionized ability and degree,the PEC performance of α-Fe2O3 increased level ranged from 8 to 20 times.Moreover,the Sn doping displayed the maximum photocurrent density of 2.0 mA/cm2 at 1.23 V vs RHE,and onset potential of the Ge doping shifted negatively from 1.25 to 0.85 V vs RHE.(2)To investigate the effect of morphology on the PEC performance of hematite,thehollow spherical nanostructure was proposed to balance the short hole diffusion length and the light absorbance.The films were prepared by template-assisted electrodeposition method,the preparation parameters were studied systematically to control the morphology,size.The hollow spherical structure improved the photocurrent density by 3 times owing to enlarging the specific surface area,improving the solar power efficiency and facilitating the holes transfer.(3)To well understand the characteristics of photoholes transfer to the hole scavengers,the Sn doped α-Fe2O3 was chosen as the model to investigate its PEC performance in the presence of H2O2,CH3OH,EDTA and NaOH.It was found that the oxygen evolution process influenced the surface states of hematite.The Von was around the peak of the Helmholtz capacitance.And the photocurrent density was dependent on the maximum of transfer resistance,which was in the order of RH2O2<RCH3OH<REDTA<ROH-.The relationship among the onset potential,photovoltage,redox potential of hole scavengers and the overpotential was established.Moreover,the surface states pinning the Fermi level were found to be related with the absorbed water molecules regardless of the reducers.(4)Based on the understanding of anisotropic electron transport in the hematite,the preparation of textured films was proposed and accomplished to enhance the PEC performance of hematite.The hydrothermal prepared α-Fe2O3 single crystalline particles were arranged along the(110)in the magnetic field.Herein,the γ was designed as the ratio of(001)in film,and it increased from 39%to 81%when magnetic field was 10 T.The positive correlation between y and photocurrent density can be assigned to the enhanced charge transport ability,the increased low-angle grain boundary and shortened charge transport distance.(5)The n-n type heterojunction of In2O3/α-Fe2O3 and Co3O4 modified n-p type heteroj unction of CaFe2O4/a-Fe2O3 were prepared and corresponding PEC performance were investigated.The PEC performance and electrochemical impedance spectra proved that the photocurrent density increased 4-10 times in the assist of heterojunction by facilitating the charge separation and extending the carriers lifetime.Furthermore,the Co3O4 which loaded on CaFe2O4/α-Fe2O3 shifted the onset potential negatively by 200 mV due to suppressing the surface states and enhancing the catalytic activity.